1. Field of the Invention
The present invention relates generally to the field of wireless communications and, in particular, to a short-range wireless radio communications system in which at least one station can communicate with at least one other station, or simultaneously with two or more other stations, and each station can handle voice and data traffic simultaneously.
2. History of the Related Art
Military and civilian (e.g., police) forces that engage routinely in mobile operations make heavy use of radio communications between personnel and vehicles. Radio transceivers typically used for this purpose are known as "simplex" or "press-to-talk" radios. For example, in a typical mobile radio communications operating environment, a party in one vehicle may need to communicate simultaneously (and independently) with two or more parties who are on foot or in other vehicles. Mobile system operators accomplish this feat by installing a separate radio in the first vehicle for each of the other parties with whom simultaneous, but independent communications will be maintained. However, operating multiple radios in a vehicle typically causes the problem known as "co-site interference," which can occur when a radio in the vehicle is transmitting one message and another radio in the vehicle is receiving a second message. Consequently, the transmitted signal overpowers or interferes with the signal being received. Currently, this problem of co-site interference is resolved by designing technically complex and expensive equipment that can compensate for the substantial interference caused by the co-located radios.
A number of conventional communications systems use Time Division Multiple Access (TDMA) for transmitting and receiving. TDMA was first developed for use in communications satellite systems, which had to support voice and data communication links between a single space-borne satellite and a large number of geographically separated earth terminals or ground stations. Prior satellite communications systems had used a relatively complicated transmission technique known as Frequency Division Multiple Access (FDMA). A problem encountered with FDMA was that the output power of the earth terminals' transmitters had to be dynamically controlled relative to each other. Specifically, the transmit power of each of the earth terminals had to be independently controlled to ensure that the energy passing through the satellite channel was fairly divided. However, the TDMA transmission techniques that have been developed (and are still being used) solved this energy division problem, by ensuring that the signal from only one earth terminal would pass through the satellite channel at any point in time.
Cellular radio telephone systems are typically operated using TDMA. For example, the so-called "Pan European Cellular System" uses TDMA to allow up to eight portable or mobile telephones to communicate simultaneously with the same base station. These telephones are duplex transceivers that transmit and receive in different frequency bands, and use time division in both bands. Each portable telephone is allocated a specific time slot in the receive band, and also a time slot in the transmit band which is offset by a fixed amount of time from the receive band. Consequently, by allocating the time slots appropriately, the mobile telephones operating in the system can transmit and receive at different points in time and thereby minimize the problem of co-site interference.
An example of the conventional use of TDMA is in the British Army's "Ptarmigan" communications system. The Ptarmigan system includes a base station and a plurality of mobile stations known as Single Channel Radio Access (SCRA) stations. The Ptarmigan system time-multiplexes the base station's transmissions. The base station broadcasts information to several of the SCRA stations, but each SCRA station is designed to select the specific information in the received signal that is intended for it. However, the SCRA stations transmit using frequency division, which avoids a requirement for the base station to time-synchronize the signals received from the SCRA stations.
As a related technical matter, the operations of transferring data (e.g., data records and files, databases, etc.) and transmitting voice information have long been considered two, separate operations. However, with the advent of such new technologies as "telematics" and "multimedia" communications, the telecommunications, computing, data and voice functions have now been integrated. For example, data and voice traffic are currently being transferred between a broad range of "terminal" types, such as telephones, radios, computers, network work stations, etc.
In the field of wireline communications, there has been a recognized need for an efficient method of simultaneously transmitting data and voice traffic. In fact, numerous attempts have been made to create a prominent standard for a combined data/voice communications protocol. One such attempt culminated in the development of the Digital Simultaneous Voice Data (DSVD) System and also a standard protocol. Nevertheless, many other proprietary, combined data/voice systems have also been developed and commercialized in the wireline communications field.
Similarly, in the field of wireless communications, there is a recognized need for a combined data/voice communications protocol. For example, while voice and computer processing operations have been successfully merged in single terminals, the terminal equipment being used has become mobile. Mobile telephones and radios, and portable computers (laptops, notebooks, etc.) have made the telecommunications and information processing technologies highly portable and thereby accentuated the need for a wireless, combined data/voice communications protocol.
A related problem is that voice and data transmission functions have different, and often contradictory, technical and functional requirements. For example, data transmissions are typically required to be error-free. Nevertheless, since the integrity of the data is much more important than the timeliness of its reception, data transmissions can tolerate substantial and variable time delays. Consequently, the existing data protocols employ re-transmission schemes, whereby the data is transmitted and re-transmitted again and again, until it is received error-free at the destination.
Conversely, voice transmissions cannot tolerate long delays, but they can tolerate a moderate amount of errors (at least until the errors become perceptible to a listener). Consequently, because of the different tolerances that data and voice transmissions have to transmission errors, different schemes will have to be used to detect and correct these errors.
Currently, studies are being conducted to determine whether (and how) data and voice traffic can be combined and transmitted simultaneously over wide-area mobile networks, such as the European Global System for Mobile Telecommunications ("GSM") and North American Digital Advanced Mobile Phone System ("D-AMPS") cellular networks. These and other existing cellular networks can support either a data link or a voice link but not both of those links at the same time.
Studies have also been conducted to determine whether local area mobile networks can carry both data and voice traffic simultaneously over relatively short ranges, but the results of these studies have not been promising. For example, wireless local area networks (WLANs) have been optimized to carry high data rate transmissions between a large number of users. Consequently, these networks do not function as well to carry voice transmissions.
Three commercially attractive frequency bands for short-range wireless communications are known as the Industrial Scientific Medical (ISM) bands. In the United States, the ISM bands are defined (by the Federal Communications Commission or FCC) at 900 MHz, 2.4 GHz and 5.7 GHz, and a license is not required to operate within these bands. Throughout the world, a license is not required to operate in the 2.4 GHz ISM band. Therefore, it will be possible to manufacture a broad range of short-range, wireless radio communications products that can be operated in these frequency bands without a license.
However, there are certain rules that ISM band users must follow. In the United States, these rules are set forth in Part 15 of the FCC's Rules, and in Europe, by the European Telecommunications Standards Institute (ETSI) in document number ETS 300 328. Unfortunately, since no operator license is required, many types of equipment are in operation that radiate signals in the ISM bands. So, signal interference from such equipment as remote telephones, baby monitors, microwave ovens, etc. can be encountered in these bands. In fact, the specific amount of interference that may be encountered is unknown, but it can be extremely hostile to radio communications. Therefore, in order to design a radio communications system that can carry data and voice traffic in the ISM bands will require careful consideration of the link protocol.